1. Field of the Invention
This invention relates generally to display technology and, more particularly, to liquid crystal displays. The invention addresses the problem of the temperature variation of the liquid crystal display characteristics when the environmental temperature can not be optimized.
2. Description of the Related Art
The transmission of light through a flat panel liquid crystal display is determined by the alignment of the liquid crystal molecules. The alignment of the liquid crystal molecules is, in turn, determined by the electric field to which the molecules are subjected. Referring to FIG. 1A, FIG. 1B, and FIG. 1C, a pictorial representation of the effect of an electric field on liquid crystal molecules of a twisted nematic liquid crystal material in a typical display environment is illustrated. Substrates 29 and 32 enclose the liquid crystal material. Polarizing elements 31, having a predetermined relative orientation, are coupled to substrates 29 and 32. Also deposited on the substrates 29 and 32 are (transparent) electrodes 30 and 34 respectively. The liquid crystal molecules 40 are enclosed between the substrates. In FIG. 1A, when a voltage, that subjects the liquid crystal molecules to an electric field with a magnitude less than a threshold value, is applied to electrodes 30 and 34, then the molecules of the liquid crystal material are not affected and light is not transmitted through the cell, i.e., through polarizing elements 31. In FIG. 1B, a voltage is applied to electrodes 30 and 34 of sufficient strength to cause complete element (of the relative orientation) of substantially all of the liquid crystal molecules 40. With this complete alignment (or saturation) of the molecules 40, light is transmitted through polarizing elements 31. In FIG. 1C, the voltage applied between electrodes 30 and 34 provides for partial alignment of the liquid crystal molecules 40 and a portion of the light applied to the cell is transmitted through polarizing elements 31.
In the prior art, a (flat panel) liquid crystal display can have a plurality of electrically separated display regions, each display region also being known as a display cell or, when the regions designate a small portion of the display, each display region is known as a pixel. Referring to FIG. 2, the apparatus for activation of the areas of a flat panel display is shown. The electrodes of each display region form a capacitor 5. A thin film transistor 20, acting as a switching element in response to control and voltage signals applied thereto, controls the charge that is stored on the plates of the capacitor and, therefore, controls the voltage and resulting electric field imposed on the liquid crystal material of the cell. When the gate terminal of transistor 20 is enabled by means of a control voltage applied to the row n conductor, the capacitor 5 is charged to the voltage applied to column m conductor. In this manner, a controllable charge is applied to each individual capacitor of the display, and hence the transmission through each individual capacitor of the display can be individually controlled.
Referring next to FIG. 3, the technique for providing a liquid crystal display for color images is shown. The substrates 29 and 32 have polarizing elements 31 with a predetermined relative orientation applied thereto and enclose the liquid crystal material 40. The substrate 32 has red, green, and blue filters 33 coupled thereto. Covering the color filters 33 is a transparent common conductor 34. Associated with each color filter region 33 is an electrode 30. The voltage applied between electrode 30 and the common electrode 34 determines the transmission of light through the liquid crystal in the region of the associated filter 33. Therefore, the intensity of the light transmitted through each color filter 33 can be controlled and the cumulative effect can be used to provide a color image.
The alignment of the liquid crystal molecules, in addition to the dependence on the applied electric field, is a function of the temperature, the temperature in turn affecting such properties as the elastic constants which in turn affect the electric field required for the alignment of the liquid crystal molecules. Referring to FIG. 4, a plot of transmission of light versus voltage (i.e., electric field) for a liquid crystal enclosed between generally parallel plates at several temperatures is illustrated.
The temperature dependence of the transmission of light through the liquid crystal flat panel cell is particularly critical in the region cf the on-set of transmission of light. In this region, relatively modest changes in temperature can result in a relatively large change in the transmission of light. However, it will be clear from review of FIG. 4 that the voltage for a selected transmission of light in one temperature range will provide a different transmission of light at a different temperature. While the effect of the change in transmission can be negligible in an on-off type of liquid crystal display when the operation is confined to the two extreme (and flat) portions of the transmission versus voltage characteristics of the liquid crystal material, in a display in which a grayscale is employed, the change in transmission can increase the difficulty of understanding the material presented by the display. In the case of color displays, the loss of accurate color tracking can compromise the benefits of providing a display with color.
A further problem can be identified with reference to FIG. 4. The grayscale characteristics of the liquid crystal display are not linear except for a limited region in the mid-transmission range. The operator will typically wish to provide an input signal from settings for a switch or similar apparatus which provide a linear scale. The conversion from a linear input signal to a signal providing a linear transmission of the liquid crystal display is typically referred to as the gamma correction to the input signal.
A need has therefore been felt for apparatus and method for providing temperature compensation for the optical transmission characteristics of a liquid crystal display. In addition, a need has been felt for apparatus and method in which the gamma correction can be combined with the temperature correction apparatus.